FIG. 1A illustrates a conventional single lens reflex (SLR) cameras involving a phase-detection autofocus (PD AF) system 5 having a dedicated sensor with a number of linear sensor pairs onto which pairs of luminous bundles from the taking lens are focused, each by its own microlens. FIG. 1B is an image of such autofocus sensor chip with a number of discrete linear sensors (horizontal and vertical black lines). Each luminous bundle focus typically covers several sensors, and each sensor pair makes up a so-called focus point, and each such point corresponds to specific position on the lens and in the frame of the image as typically shown in the image finder. Over the years, more and more advanced sensors and autofocus points have been added to standard cameras, such as cross-type and oblique serving to improve autofocus on a wide range of different structures. Also, it has been suggested to improve the accuracy by expanding the light bundles onto area sensors such as in U.S. Pat. No. 6,297,909.
Modern phase-detection AF systems suffers from a number of disadvantages.
The optical elements and sensors require high precision to get accurate results, and must be properly installed and aligned during the manufacturing process. Even slight deviations would result in the autofocus being off. Camera manufacturers have high precision calibration systems that allows for calibration of the autofocus for each individual camera during the inspection and quality assurance process. Even when precisely calibrated, the precision degrades with changes in temperature.
In low light conditions, the signal to noise ratio in the readouts from the sensors becomes small and the comparison between the phase signals becomes difficult and slow.
Cross type autofocus points provides precise autofocus on horizontal and vertical structures. But structures oriented at for example 45 degrees will be smeared out on both the vertical and the horizontal line sensor of the cross, resulting in poor precision. Some cameras have special autofocus points for oblique structures, but since each region in the frame can only be covered by one autofocus point, the specific structure to be focused on may not overlap with an autofocus point suitable for the shape and orientation of this structure.
Recently sensors with phase-detection pixels built into the sensor so that the image taking sensor can be used for both phase- and contrast-detection AF has been introduced, in particular by Canon, see e.g. WO 2010/101096. These are typically referred to as Dual Pixel CMOS AF or Hybrid CMOS AF and are used in some DSLR cameras to improve autofocus during in live view and video mode. Such DSLR cameras then also has a separate phase-detection AF module placed under the mirror which is used in other modes.
In the last appr. 6 years, the so-called compact system cameras, CSC, (or mirrorless cameras or Electronic viewfinder interchangeable lens (EVIL)) have been introduced. Since these do not have a mirror they cannot use conventional separate phase-detection AF modules. More advanced CSCs use the Dual Pixel CMOS AF or Hybrid CMOS AF systems to provide phase-detection AF.
DSLR or CSC cameras with dual pixel or hybrid AF systems do not provide a replacement of separate phase-detection AF modules in DSLR cameras, and there is still a need for improving separate phase-detection AF systems.
Hence, an improved autofocus system would be advantageous, and in particular a more precise, flexible and/or reliable autofocus system would be advantageous.